Cast product having alumina barrier layer

ABSTRACT

The present invention provides a cast product that can further enhance the stability of an alumina barrier layer and can exhibit further superior oxidation resistance, carburization resistance, nitriding resistance, corrosion resistance, and the like when used under a high-temperature atmosphere. The cast product according to the present invention is a cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, and the aluminum oxide is (Al (1-x) M (x) ) 2 O 3 , where M is at least one of Cr, Ni, Si, and Fe, and x satisfies a relationship 0&lt;x&lt;0.5. Furthermore, the cast product according to the present invention is a cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, and at least one of Cr, Ni, Si, and Fe forms a solid solution in the aluminum oxide, and at least one of Cr, Ni, Si, and Fe forming the solid solution with Al is contained so as to satisfy a relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a cast product having an aluminabarrier layer, and more specifically, to a cast product having analumina barrier layer with a stable structure.

Description of the Related Art

Heat-resistant steel castings such as reaction tubes and decompositiontubes for producing ethylene, and hearth rolls, radiant tubes and metaldusting-resistant materials for use in carburizing heat-treatmentfurnaces are exposed to a high-temperature atmosphere, and therefore aremade of an austenite-based heat-resistant alloy having superiorhigh-temperature strength.

A metal oxide layer is formed on the surface of this type ofaustenite-based heat-resistant alloy during use in a high-temperatureatmosphere. This oxide layer serves as a barrier, and thus protects thebase material under a high-temperature atmosphere.

On the other hand, when Cr-oxides (mainly constituted by Cr₂O₃) areformed as the metal oxide, the Cr-oxide layer has an insufficientfunction for preventing the entry of oxygen and carbon due to its lowdenseness, thus causing the internal oxidation under a high-temperatureatmosphere and the thickening of the oxide film. Moreover, the Cr-oxidelayer is likely to become detached during repeated cycles of heating andcooling. Even if the Cr-oxide layer do not become detached, the Cr-oxidelayer has an insufficient function for preventing the entry of oxygenand carbon from an outside atmosphere, and therefore, there is adisadvantageous situation in which oxygen and carbon pass through thefilm and cause the internal oxidation or carburization of the basematerial.

To address this, it is proposed that an oxide layer including alumina(Al₂O₃) as a main component that has high denseness and makes itdifficult for oxygen and carbon to pass therethrough is formed on thesurface of the base material by increasing the content of Al comparedwith that in a common austenite-based heat-resistant alloy (see PatentDocuments 1 and 2, for example).

However, Al is a ferrite-forming element, and therefore, when thecontent of Al is increased, the ductility of the materials isdeteriorated and the high-temperature strength is reduced. This tendencyof reduction of the ductility is observed particularly when the contentof Al exceeds 5%. For this reason, the austenite-based heat-resistantalloy of Patent Documents 1 and 2 can be expected to have an enhancedbarrier function due to Al₂O₃, but has the disadvantage of causing areduction of the ductility of the base material.

Therefore, in order to provide a cast product that can secure thehigh-temperature stability of Al₂O₃ and can achieve a superior barrierfunction under a high-temperature atmosphere without reducing theductility of the materials, Patent Document 3 proposes a cast product inwhich an alumina barrier layer including Al₂O₃ is formed on the innersurface of a cast body and Cr-based particles that contain Cr at ahigher concentration than that of a matrix of the base material aredispersed at an interface between the alumina barrier layer and the castbody by performing heat treatment under an oxidizing atmosphere afterprocessing the inner surface such that a surface roughness (Ra) of thecast body is 0.05 to 2.5 μm (see Patent Document 3, for example).

Due to the presence of a stable alumina barrier layer, superioroxidation resistance, carburization resistance, nitriding resistance,corrosion resistance, and the like of the cast product of Patentdocument 3 can be maintained for a long period of time of use under ahigh-temperature atmosphere.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP S52-78612A

Patent Document 2: JP S57-39159A

Patent Document 3: WO 2010/113830

It is an object of the present invention to provide a cast product thatcan further enhance the stability of the alumina barrier layer and canexhibit further superior oxidation resistance, carburization resistance,nitriding resistance, corrosion resistance, and the like when used undera high-temperature atmosphere.

SUMMARY OF THE INVENTION

The cast product according to the present invention is a cast producthaving an alumina barrier layer including an aluminum oxide on a surfaceof a matrix, wherein the aluminum oxide is (Al_((1-x))M_((x)))₂O₃, whereM is at least one of Cr, Ni, Si, and Fe, and x satisfies a relationship0<x<0.5.

Also, the cast product according to the present invention is a castproduct having an alumina barrier layer including an aluminum oxide on asurface of a matrix, wherein at least one of Cr, Ni, Si, and Fe forms asolid solution in the aluminum oxide, and the at least one of Cr, Ni,Si, and Fe forming the solid solution with Al is contained so as tosatisfy a relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

Effects of the Invention

With the cast product of the present invention, at least one of Cr, Ni,Si, and Fe forms a solid solution in an alumina barrier layer formed ona surface of a matrix, thus enabling an aluminum oxide phase to have astable structure. With the aluminum oxide, it is possible to suppressthe coupling between the matrix and oxygen and the formation of oxidescontaining Cr, Ni, Si, Fe and the like as a main component on thesurface of the matrix.

This makes it possible that the cast product of the present inventionexhibits further superior oxidation resistance, carburizationresistance, nitriding resistance, corrosion resistance, and the likewhen used under a high-temperature atmosphere.

Accordingly, when the cast product of the present invention is used fora reaction tube for producing ethylene, for example, it is possible tosuppress the occurrence of coking, to prevent the yield from beingreduced by the reduction of heat exchange rate and thermal conductivitydue to the occurrence of coking, and to extend continuous operatingtime. In addition, since coking is unlikely to occur, it is possible toreduce the frequency and time period for coking-removing operation andto enhance operation efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cast product before heattreatment.

FIG. 2 is a schematic cross-sectional view illustrating a state where adilute-Al layer is formed by low-temperature heat treatment.

FIG. 3 is a schematic cross-sectional view illustrating a state where aconcentrated-Al layer is formed between the dilute-Al layer and a matrixby high-temperature heat treatment.

FIG. 4 shows a TEM photograph of a film of Working Example 2 and graphsillustrating the results of an EDX analysis.

FIG. 5 shows a TEM photograph of a film of Working Example 7 and graphsillustrating the results of an EDX analysis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail.

A cast product of the present invention has an alumina barrier layerincluding an aluminum oxide on the surface of a matrix.

The aluminum oxide in the alumina barrier layer is(Al_((1-x))M_((x)))₂O₃, where M is at least one of Cr, Ni, Si, and Fe,and x is adjusted so as to satisfy the relationship 0<x<0.5.

Also, at least one of Cr, Ni, Si, and Fe forms a solid solution in thealuminum oxide in the alumina barrier layer, and at least one of Cr, Ni,Si, and Fe forming a solid solution with Al is contained so as tosatisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

Explanation of Reasons for Limiting Components

As long as the cast product of the present invention is made of aheat-resistant alloy containing Cr in an amount of 15 mass % or more, Niin an amount of 18 mass % or more, and Al in an amount of 1 to 5 mass %,it is possible to obtain the effects of the present invention. The castproduct of the present invention is made of the following components,for example. It should be noted that in the following description, theterm “%” refers to “mass %” unless otherwise stated.

C: 0.05 to 0.7%

C acts to improve castability and enhance a high-temperature creeprupture strength. Therefore, the content of C is set to at least 0.05%.However, if the content is too large, a primary carbide of Cr₇C₃ islikely to be extensively formed and the movement of Al for forming thealumina barrier layer is suppressed. As a result, Al is insufficientlysupplied to the surface portion of a cast body and the alumina barrierlayer is locally divided, and thus the continuity of the alumina barrierlayer is impaired. Moreover, a secondary carbide excessively deposits toreduce ductility and toughness. Therefore, the upper limit is set to0.7%. It should be noted that the content of C is more desirably 0.3 to0.5%.

Si: more than 0% to 2.5% or less

Si is contained to serve as a deoxidizer for molten alloy and to enhancethe fluidity of molten alloy. If the content is too large, ahigh-temperature creep rupture strength is reduced, and therefore, theupper limit is set to 2.5%. It should be noted that the content of Si ismore desirably 2.0% or less.

Mn: More than 0% to 3.0% or Less

Mn is contained to serve as a deoxidizer for molten alloy and to fix Sin molten alloy. If the content is too large, high-temperature creeprupture strength is reduced, and therefore, the upper limit is set to3.0%. It should be noted that the content of Mn is more desirably 1.6%or less.

Cr: 15.0 to 50.0%

Cr is contained in an amount of 15.0% or more in order to contribute tothe enhancement of high-temperature strength and cyclic oxidationresistance. However, if the content is too large, high-temperature creeprupture strength is reduced, and therefore, the upper limit is set to50.0%. It should be noted that the content of Cr is more desirably 23.0to 35.0%.

Ni: 18.0 to 70.0%

Ni is an element that is necessary to secure cyclic oxidation resistanceand the stability of a metal structure. If the content of Ni is small,the content of Fe relatively becomes large. As a result, a Cr—Fe—Mnoxide is likely to be formed on the surface of the cast body, and thusthe formation of the alumina barrier layer is inhibited. Therefore, thecontent of Ni is set to at least 18.0%. Even if the content of Niexceeds 70.0%, it is impossible to obtain the efficacy corresponding tothe increasing amount, and therefore, the upper limit is set to 70.0%.It should be noted that the content of Ni is more desirably 28.0 to45.0%.

Al: 1.0 to 5.0%

Al is an element that is effective for enhancing carburizationresistance and coking resistance. Also, in the present invention, Al isan element that is essential for forming the alumina barrier layer onthe surface of the cast body. Therefore, the content of Al is set to atleast 1.0%. However, if the content of Al exceeds 5%, the ductility isdeteriorated, and therefore, the upper limit is set to 5.0% in thepresent invention. It should be noted that the content of Al is moredesirably 2.5 to 3.8%.

Rare Earth Elements: 0.005 to 0.4%

The term “rare earth elements” means 17 elements including 15 elementsof the lanthanide series ranging from La to Lu in the periodic table,and Y and Sc. It is preferable that rare earth elements to be containedin the heat-resistant alloy of the present invention include at leastone element selected from the group consisting of Ce, La and Nd. Rareearth elements contribute to the formation of the alumina barrier layerand the enhancement of stability thereof.

When the alumina barrier layer is formed by heat treatment under ahigh-temperature oxidizing atmosphere, rare earth elements that arecontained in an amount of 0.005% or more effectively contribute to theformation of the alumina barrier layer.

On the other hand, if the content is too large, the ductility andtoughness are deteriorated, and therefore, the upper limit is set to0.4%.

W: 0.5 to 10.0% and/or Mo: 0.1 to 5.0%

W and Mo enhance creep rupture strength by forming a solid solution in amatrix and strengthening an austenite phase. At least one of W and Mo iscontained in order to achieve this efficacy. The content of W is set to0.5% or more, and the content of Mo is set to 0.1% or more.

However, if the contents of W and Mo are too large, ductility is reducedand carburization resistance is deteriorated. Moreover, as in the casewhere the content of C is large, a primary carbide of (Cr, W, Mo)₇C₃ islikely to be extensively formed and the movement of Al for forming thealumina barrier layer is suppressed. As a result, Al is insufficientlysupplied to the surface portion of the cast body and the alumina barrierlayer is locally divided, and thus the continuity of the alumina barrierlayer is likely to be impaired. Furthermore, since W and Mo have a largeatomic radius, they suppress the movement of Al and Cr and inhibit theformation of the alumina barrier layer due to the formation of a solidsolution in the matrix.

Therefore, the content of W is set to 10.0% or less, and the content ofMo is set to 5.0% or less. It should be noted that when both elementsare contained, the total content is preferably set to 10.0% or less.

In addition, the following components can be contained.

At least one selected from the group consisting of Ti in an amount of0.01 to 0.6%, Zr in an amount of 0.01 to 0.6%, and Nb in an amount of0.1 to 1.8%

Ti, Zr and Nb are elements that are likely to form carbides, and formless solid solutions in the matrix than W and Mo. Therefore, Ti, Zr andNb do not exhibit any particular action of forming the alumina barrierlayer, but enhance creep rupture strength. At least one of Ti, Zr and Nbcan be contained as needed. The content of Ti or Zr is set to 0.01% ormore and the content of Nb is set to 0.1% or more.

However, if they are excessively added, ductility is reduced.

Furthermore, Nb reduces the peeling resistance of the alumina barrierlayer. Therefore, the upper limit of the content of Ti or Zr is set to0.6%, and the upper limit of the content of Nb is set to 1.8%.

B: more than 0% to 0.1% or less

Since B exhibits an action of strengthening the particle boundaries ofthe cast body, B can be contained as needed. It should be noted that ifthe content of B is large, creep rupture strength is reduced, andtherefore, the content of B is set to 0.1% or less even in the casewhere B is added.

The heat-resistant alloy making up the cast body of the presentinvention includes the above-described components and Fe as the balance.P, S, and other impurities that are inevitably mixed in the alloy whenmelting the alloy may be contained as long as such impurities arecontained in an amount within a range that is usually allowable to thistype of alloy material.

Cast Product

Molten metal having a composition including the above-describedcomponents is produced and cast by centrifugal casting, static casting,or the like into the cast product of the present invention having theabove composition.

The obtained cast product can be shaped depending on the intendedapplication.

One example of the cast product is a tube, in particular, a reactiontube used under a high-temperature environment.

It is particularly preferable to produce the cast product of the presentinvention by centrifugal casting. This is because when using centrifugalcasting, a fine metal structure grows in the radial direction withorientation due to the progress of cooling by a metal mold and thus analloy structure in which Al easily moves can be obtained.

Heat treatment, which will be described later, is performed on the castproduct. The alumina barrier layer having a stable phase structure isformed by the heat treatment.

Heat Treatment

The heat treatment is performed on the cast product of the presentinvention under an oxidizing atmosphere. The heat treatment can bedivided into low-temperature heat treatment and high-temperature heattreatment. It should be noted that the low-temperature heat treatmentand the high-temperature heat treatment can be performed in separatesteps or the high-temperature heat treatment may be performedsubsequently to the low-temperature heat treatment.

Low-Temperature Heat Treatment

The low-temperature heat treatment is treatment in which an aluminumoxide layer is formed on the surface of the matrix under an oxidizingatmosphere. One example of a low temperature is a temperature of lessthan 1050° C. It is desirably 600 to 900° C. It is desirable to performthe low-temperature heat treatment for 5 to 15 hours.

By performing the low-temperature heat treatment, oxygen comes intocontact with a matrix 10 as shown in. FIG. 1 to oxidize Al, Cr, Ni, Si,and Fe that has diffused from the matrix 10 to the surface of thematrix, and an oxide layer 22 is formed as shown in FIG. 2. Since thisheat treatment is performed at a low temperature, Al forms oxides priorto Cr, Ni, Si, and Fe. Accordingly, an aluminum oxide layer 22 thatcontains Al as a main component and in which at least one of Cr, Ni, Si,and Fe, which have similarly diffused from the matrix, forms a solidsolution is the oxide layer.

In the aluminum oxide formed by the low-temperature heat treatment, atleast one of Cr, Ni, Si, and Fe forming a solid solution with Al iscontained so as to satisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in anatomic % ratio. In addition, it is desirable that the compositionthereof is (Al_((1-x))M_((x)))₂O₃, where M is at least one of Cr, Ni,Si, and Fe, and x satisfies the relationship 0<x<0.5. Moreover, at leastCr forms a solid solution in the aluminum oxide, and Cr forming a solidsolution with Al is more preferably contained so as to satisfy therelationship Al/Cr≧10 in atomic % ratio, and still more preferably therelationship Al/Cr≧15. Furthermore, at least one of Ni, Si, and Fe formsa solid solution, and it is more desirable that the total atomic % of atleast one of Ni, Si, and Fe forming a solid solution with Al is 10 atm %or less.

The aluminum oxide formed by the above-described low-temperature heattreatment has a metastable γ or θ alumina structure, which is a porousstructure. Accordingly, the strength is not enough.

High-Temperature Heat Treatment

The high-temperature heat treatment is heat treatment that is performedafter the low-temperature heat treatment, and in which, as describedlater, the phase of the aluminum oxide formed by the low-temperatureheat treatment is transformed to an α alumina structure (corundumstructure), and an aluminum oxide layer containing Al at a highconcentration is formed between that aluminum oxide layer and thematrix.

The high-temperature heat treatment can be performed by heating the castproduct on which the low-temperature heat treatment has been performedand the alumina barrier layer having a γ or θ alumina structure has beenformed, at a high temperature under an oxidizing atmosphere. One exampleof a high temperature is a temperature of 1050° C. or more. It isdesirable to perform the high-temperature heat treatment for 3 to 15hours.

By performing the high-temperature heat treatment, the phase of thealuminum oxide that was formed first and has a γ or θ alumina structureis transformed to a stable a alumina structure (corundum structure). Inthe present invention, at least one of Cr, Ni, Si, and Fe forms a solidsolution in the aluminum oxide layer having a γ or θ alumina structure.This makes it possible to promote the phase transformation from a γ or θalumina structure to an α alumina structure (corundum structure)compared with the case where the aluminum oxide layer contains Al in ahigh ratio.

Then, the high-temperature heat treatment is further continuouslyperformed on the cast product having the aluminum oxide layer in whichthe phase has been transformed to an α alumina structure (corundumstructure), and thus oxygen passes through the aluminum oxide layer 22as shown in FIG. 3.

The oxygen that has passed through the aluminum oxide layer 22 oxidizesAl that diffuses from the matrix, and an aluminum oxide layer 24 thatcontains Al at a high concentration is formed.

Here, as shown in FIG. 3, the aluminum oxide layer that is formed by thelow-temperature heat treatment and in which at least one of Cr, Ni, Si,and Fe forms a solid solution is referred to as “dilute-Al layer”, andthe aluminum oxide layer that is formed between the dilute-Al layer andthe surface of the matrix and contains Al at a high concentration isreferred to as “concentrated-Al layer”. Specifically, in theconcentrated-Al layer 24, Al/(Cr+Ni+Si+Fe) is larger than that in thedilute-Al layer 22.

It is thought that the reason why in terms of the alumina barrier layer,the concentrated-Al layer formed between the matrix and the dilute-Allayer contains Al at a higher concentration than the dilute-Al layer atthe surface is as follows.

The formed dilute-Al layer 22 allows a small amount of oxygen to passtherethrough under an oxidizing atmosphere.

Al, Cr, Ni, Si, and Fe diffuse from the matrix 10 side to the matrixsurface side as shown in FIG. 3. However, since Al needs a smalleramount of energy for binding to oxygen than Cr, Ni, Si, and Fe, Alpriorly binds to oxygen, and the aluminum oxide layer (concentrated-Allayer 24) containing aluminum oxide at a high concentration is formedbetween the matrix 10 and the dilute-Al layer 22.

The concentrated-Al layer 24 is formed by high-temperature heattreatment, and thus has a stable a alumina structure (corundumstructure). It is desirable that 80 vol % or more of a crystal structureof each of the dilute-Al layer 22 and the concentrated-Al layer 24 is anα alumina structure (corundum structure).

Since the alumina barrier layer 20 is constituted by the dilute-Al layer22 and the concentrated-Al layer 24 formed between the matrix 10 and thedilute-Al layer 22, both of which have stable a alumina structures(corundum structures), the alumina barrier layer 20 has high denseness,serves as a barrier for preventing oxygen, carbon and nitrogen fromentering the base material from the outside during use under ahigh-temperature atmosphere in the cast product provided therewith, andcan maintain superior oxidation resistance, carburization resistance,nitriding resistance, corrosion resistance, and the like for a longperiod of time.

It should be noted that the concentrated-Al layer 24 is desirably formedso as to be thicker than the dilute-Al layer 22, and it is preferable toform the concentrated-Al layer 24 such that the thickness of theconcentrated-Al layer 24 is one fifth or more of that of the aluminabarrier layer 20.

It is more desirable that the dilute-Al layer 22 has a thickness of 0.04to 8.0 μm and the concentrated-Al layer 24 has a thickness of 0.01 to2.0 μm.

It is desirable to heat the cast product while rotating it in order topreferably form the aluminum oxide layer in the above-describedlow-temperature heat treatment and high-temperature heat treatment. Thismakes it possible to heat the cast product uniformly and bring the castproduct into contact with oxygen in a good state. As a result, it ispossible to reduce the surface roughness (Ra) of the formed aluminabarrier layer 20.

Surface Treatment

It is possible to perform surface treatment on the alumina barrier layerof the cast product as needed. One example of the surface treatment ispolishing. For example, when the cast product is used for a reactiontube, Fe, Ni and the like of the cast product come into contact with ahydrocarbon as a raw material and coke (carbon) is likely to adhere tothe inner surface of the tube due to the catalytic action of Fe and Ni,but it is possible to suppress the adhesion of coke by performing thesurface treatment to reduce the surface roughness (Ra) of the aluminabarrier layer.

It is desirable to perform the surface treatment such that the surfaceroughness (Ra) of the alumina barrier layer is 15 μm or less. Thesurface roughness (Ra) is more desirably 0.05 to 10 μm.

Example 1

Molten metal was produced by atmospheric melting in a high-frequencyinduction melting furnace and cast by metal mold centrifugal castinginto tube bodies having alloy chemical compositions shown in Table 1below. The tube body has an inner diameter of 80 mm, an outer diameterof 100 mm, and a length of 250 mm.

TABLE 1 Alloy chemical composition (the balance includes Fe andinevitable impurities) (mass %) No. C Si Mn Cr Ni Al REM W Mo Ti Zr Nb BWork. 0.33 0.49 0.32 24.5 43.6 1.2 0.25 2.1 0.6 0.7 Ex. 1 Work. 0.450.49 0.9 24.3 34.6 1.0 0.16 1.5 0.13 Ex. 2 Work. 0.4 0.33 0.7 23.8 31.53.3 0.26 2.8 0.03 Ex. 3 Work. 0.46 1.5 1.2 25.2 35.0 2.8 0.21 4.2 0.090.12 Ex. 4 Work. 0.26 0.41 0.5 23.5 34.6 3.1 0.07 0.9 0.07 Ex. 5 Work.0.31 0.4 0.2 18.3 67.1 4.7 0.01 0.4 1.6 Ex. 6 Work. 0.38 0.26 0.4 23.834.4 4.9 0.11 0.95 0.1 Ex. 7 Work. 0.67 1.5 1.1 23.9 40.1 4.9 0.19 2.90.03 Ex. 8 Comp. 0.33 1.78 0.17 25.0 33.4 0.0 0.11 0.83 0.12 Ex. 1 Comp.0.40 1.3 0.9 25.4 15.0 0.9 0.29 2.9 Ex. 2 Comp. 0.27 1.02 0.2 23.8 33.61.1 0.19 11.7 Ex. 3 Comp. 0.34 0.6 0.2 25.0 45.4 2.9 0.09 1.5 1.3 Ex. 4Comp. 0.45 1.43 1.3 22.9 34.7 3.2 0.24 3.15 0.23 Ex. 5 Comp. 0.45 0.540.7 23.8 29.7 5.1 0.15 1.5 0.21 Ex. 6

Two steps of heat treatment that differed in a heating temperature wereperformed on each of Working Examples 1 to 8, which were obtainedexamples of the present invention, and Comparative Examples 1 to 6 underan oxidizing atmosphere. First, the low-temperature heat treatment wasperformed, and the high-temperature heat treatment was subsequentlyperformed. The low-temperature heat treatment was performed for 5 hours,and the high-temperature heat treatment was performed for 5 hours.

TABLE 2 Low-temperature heat High-temperature heat treatment temperaturetreatment temperature No. (° C.) (° C.) Work. Ex. 1 700 1050 Work. Ex. 2900 1100 Work. Ex. 3 800 1050 Work. Ex. 4 800 1100 Work. Ex. 5 900 1100Work. Ex. 6 600 1050 Work. Ex. 7 700 1100 Work. Ex. 8 600 1150 Comp. Ex.1 800 900 Comp. Ex. 2 1000 1250 Comp. Ex. 3 1200 1300 Comp. Ex. 4 5001150 Comp. Ex. 5 900 1000 Comp. Ex. 6 600 800

The atomic percentages of elements (Al, Cr, Fe, Ni, Si, O) contained inthe alumina barrier layer formed on the surface of a sample tube of eachof Working Examples 1 to 8 and Comparative Examples 1 to 6 that had beensubjected to the heat treatment were measured by an EDX analysis (energydispersive X-ray spectrometry). Table 3 shows the results.

TABLE 3 Concentrated-Al Cr + Fe + Al/ layer thickness/ Al Cr Fe Ni Si ONi + Si (Cr + Fe + Fe + Ni + Si Al barrier layer No. (atm %) (atm %)(atm %) (atm %) (atm %) (atm %) (atm %) Ni + Si) Al/Cr (atm %) thicknessWork. 41.66 2.7 4.46 3.52 48.56 10.7 3.9 15.43 7.98 0.50 Ex. 1 Work.39.2 3.6 1.4 3.2 52.6 8.2 4.8 10.89 4.6 0.30 Ex. 2 Work. 46.6 0.1 0.153.2 0.2 233.0 466.0 0.1 0.80 Ex. 3 Work. 40.96 2.23 2.34 2.12 52.35 6.76.1 18.37 4.46 0.60 Ex. 4 Work. 38.4 1.5 10.1 6.4 1.2 42.4 19.2 2.025.60 17.7 0.40 Ex. 5 Work. 42.49 1.31 2.13 1.95 52.12 5.4 7.9 32.444.08 0.75 Ex. 6 Work. 42.69 1.28 55.93 1.3 33.4 33.35 0 0.70 Ex. 7 Work.41.8 2.4 3.9 6.5 45.4 12.8 3.3 17.42 10.4 0.50 Ex. 8 Comp. 4.61 10.257.09 31 47.05 53.0 0.0 0.00 48.34 0.00 Ex. 1 Comp. 11.17 15.25 30.125.28 1.47 16.73 72.1 0.2 0.73 56.85 0.10 Ex. 2 Comp. 6.7 35.46 1.65 1.81.64 52.75 40.6 0.2 0.19 5.09 0.05 Ex. 3 Comp. 36.7 6.7 1.8 10.3 44.518.8 1.95 5.48 12.1 0.15 Ex. 4 Comp. 9.77 21.54 11.34 9.51 1.02 46.8243.4 0.2 0.45 21.87 0.10 Ex. 5 Comp. 8.35 11 15.18 13.76 51.71 39.9 0.20.76 28.94 0.05 Ex. 6

All of Working Examples 1 to 8, which are examples of the presentinvention, satisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in atomic %ratio. Furthermore, they satisfy the relationship Al/Cr≧10. On the otherhand, Comparative Example 1 contains no Al in the matrix, and therefore,no aluminum oxide is formed and both Al/(Cr+Ni+Si+Fe) and Al/Cr arezero.

Moreover, all of Comparative Examples 2 to 6 satisfy the relationshipsAl/(Cr+Ni+Si+Fe)<2.0 and Al/Cr<10.

Furthermore, Fe+Ni+Si is 10 atm % or less in Working Examples 1 to 4,and 6 and 7, and Comparative Example 3, and more than 10 atm % in otherworking examples and comparative examples.

The ratio of the thickness of the concentrated-Al layer with respect tothe thickness of the formed alumina barrier layer was measured in eachof obtained Working Examples 1 to 8 and Comparative Examples 1 to 6.Table 3 above shows the results.

Table 3 shows that the ratio of the thickness of the concentrated-Allayer with respect to the thickness of the alumina barrier layer is 0.3or more, that is, one fifth or more in each working example, but it is0.15 at most in the comparative examples. It should be noted that sinceComparative Example 1 contains no Al, no alumina barrier layer isformed.

This shows that since the low-temperature heat treatment was performedon the working examples, which were examples of the present invention,at a temperature of less than 1050° C. and the high-temperature heattreatment was performed thereon at a temperature of 1050° C. or more,the dilute-Al layer was formed on the surface of the matrix by thelow-temperature heat treatment and then the concentrated-Al layer couldbe formed between the dilute-Al layer and the matrix by thehigh-temperature heat treatment.

On the other hand, it is thought that the ratio of the thickness of theconcentrated-Al layer with respect to the thickness of the aluminabarrier layer was 0.15 at most in Comparative Examples 2 to 6 on whichthe alumina barrier layer was formed for the following reasons.

In Comparative Example 2, the cast body contained Al in a small amountof 0.9%, and Al for forming a film on the surface of the cast body wasinsufficient. In Comparative Example 3, since the low-temperature heattreatment was performed at a high temperature of 1200° C., oxidescontaining Cr, Ni, Si, Fe and the like as a main component were formedbefore the alumina barrier layer having a γ or θ alumina structure wasformed. In Comparative Example 4, since the low-temperature heattreatment was performed at a low temperature of 500° C., no aluminabarrier layer having a γ or θ alumina structure was formed. InComparative Examples 5 and 6, the high-temperature heat treatment wasperformed at a low temperature of 1000° C. As a result, a small amountof oxygen passed through the dilute-Al layer in the high-temperatureheat treatment after the dilute-Al layer was formed in thelow-temperature heat treatment, and Al did not obtain sufficient energyfor binding to the oxygen taken in because the temperature was low.

Next, a coking test was performed on the obtained sample tubes.

The coking test was performed by placing the sample tubes in an electricfurnace, supplying a hydrocarbon (ethane) to the sample tubes, and thenheating them at a high temperature (955° C.) for a predetermined time(12 to 24 hours). After the test, the degrees of carburization of theinner surfaces of the sample tubes were compared, and the weight ratioof coke (carbon) adhering to the inner surface of each sample tube wasmeasured. Table 4 shows the results.

TABLE 4 Carburization Weight ratio of Surface No. resistance formed cokeroughness (Ra) Work. Ex. 1 Good 0.4 0.13 Work. Ex. 2 Fair 0.6 5.51 Work.Ex. 3 Good 0.8 7.63 Work. Ex. 4 Good 0.6 1.53 Work. Ex. 5 Fair 1.0 11.7Work. Ex. 6 Good 0.6 1.54 Work. Ex. 7 Good 0.7 2.02 Work. Ex. 8 Good 1.113.8 Comp. Ex. 1 Poor 0.9 9.17 Comp. Ex. 2 Poor 0.7 4.5 Comp. Ex. 3 Poor1.5 18.1 Comp. Ex. 4 Poor 0.7 8.32 Comp. Ex. 5 Poor 0.8 6.27 Comp. Ex. 6Poor 1.4 16.47

Table 4 shows that all of Working Examples 1 to 8, which were examplesof the present invention, had favorable carburization resistance. On theother hand, all of the comparative examples were carburized to theinside of the sample tube.

It is because the alumina barrier layer having a stable a aluminastructure (corundum structure) that was constituted by theconcentrated-Al layer and the dilute-Al layer was preferably formed onthe surface of the matrix that Working Examples 1 to 8 had superiorcarburization resistance. In particular, Working Examples 1, 3, 4, and 6to 8 had extremely superior carburization resistance compared with theother working examples. It is thought that this is because a smalleramount of the concentrated-Al layer was formed in. Working Examples 2and 5 than in the other working examples.

Furthermore, the surface roughness (Ra) of each sample tube wasmeasured. Table 4 shows the results as well. Table 4 shows that theweight ratio of the formed coke and the surface roughness (Ra) weresubstantially in proportion to each other. Accordingly, the surfaceroughness (Ra) is preferably 15 μm or less, and more preferably 10 μm orless.

It is possible to adjust the surface roughness (Ra) by performing theheat treatment while rotating the cast product. It is thought that it isbecause the heat treatment for forming a film was not properly performedand the surface roughness was increased due to the peeling andrestoration of the film that the surface roughness of ComparativeExamples 3 and 6 exceeded 15 μm.

Example 2

The alumina barrier layers of Inventive Examples 2 and 7 are observedusing a transmission electron microscope (TEM). Furthermore, the EDXanalysis was performed on the dilute-Al layer and the concentrated-Allayer of each inventive example. FIG. 4 shows the results from InventiveExample 2, and FIG. 5 shows the results from Inventive Example 7.

FIG. 4 shows that in Inventive Example 2, small amounts of Cr, Fe, andNi were detected in the dilute-Al layer 22 formed on the surface side,which contained Al oxides as a main component. On the other hand, no Cr,Fe, Ni, and the like were detected in the concentrated-Al layer 24 butAl. Accordingly, it is found that the concentrated-Al layer 24 was madeof aluminum oxide having a very high purity.

FIG. 5 shows that in Inventive Example 7, a small amount of Cr wasdetected in the dilute-Al layer 22 formed on the surface side, whichmainly included Al oxides. On the other hand, nothing but Al wasdetected in the concentrated-Al layer 24. Accordingly, it is found thatthe concentrated-Al layer 24 was made of aluminum oxide having a veryhigh purity.

1-12. (canceled)
 13. A cast product having an alumina barrier layerincluding an aluminum oxide on a surface of a matrix, wherein thealuminum oxide is (Al_((1-x))M_((x)))₂O₃, where M is at least one of Cr,Ni, Si, and Fe, and x satisfies a relationship 0<x<0.5.
 14. The castproduct according to claim 13, wherein 80 vol % or more of a crystalstructure of the aluminum oxide is a corundum structure.
 15. The castproduct according to claim 13, wherein at least Cr forms a solidsolution in the aluminum oxide, and the Cr forming the solid solutionwith Al is contained so as to satisfy a relationship Al/Cr≧10 in anatomic % ratio.
 16. The cast product according to claim 13, wherein atleast one of Ni, Si, and Fe forms a solid solution in the aluminumoxide, and a total atomic % of the at least one of Ni, Si, and Feforming the solid solution with Al is 10 atm % or less.
 17. The castproduct according to claim 13, wherein the aluminum oxide has aconcentrated-Al layer in which the Al/(Cr+Ni+Si+Fe) on the matrix sideis larger than that on the surface side.
 18. The cast product accordingto claim 17, wherein a thickness of the concentrated-Al layer is onefifth or more of a thickness of the alumina barrier layer.
 19. The castproduct according to claim 13, wherein the alumina barrier layer has asurface roughness (Ra) of 15 μm or less.
 20. The cast product accordingto claim 13, wherein the matrix includes C in an amount of 0.05 to 0.7mass %, Si in an amount of more than 0 mass % to 2.5 mass % or less, Mnin an amount of more than 0 mass % to 3.0 mass % or less, Cr in anamount of 15.0 to 50.0 mass %, Ni in an amount of 18.0 to 70.0 mass %,Al in an amount of 1.0 to 5.0 mass %, rare earth elements in an amountof 0.005 to 0.4 mass %, and Win an amount of 0.5 to 10.0 mass % and/orMo in an amount of 0.1 to 5.0 mass %, and the balance includes Fe andinevitable impurities.
 21. The cast product according to claim 20,wherein the matrix further includes at least one selected from the groupconsisting of Ti in an amount of 0.01 to 0.6 mass %, Zr in an amount of0.01 to 0.6 mass %, and Nb in an amount of 0.1 to 1.8 mass %.
 22. Thecast product according to claim 20, wherein the matrix further includesB in an amount of more than 0 mass % to 0.1 mass % or less.
 23. Areaction tube made of the cast product according to claim 13, whereinthe alumina barrier layer is formed on an inner surface of the tubethrough which hydrocarbon gas as a raw material flows.
 24. A castproduct having an alumina barrier layer including an aluminum oxide on asurface of a matrix, wherein at least one of Cr, Ni, Si, and Fe forms asolid solution in the aluminum oxide, and the at least one of Cr, Ni,Si, and Fe forming the solid solution with Al is contained so as tosatisfy a relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio. 25.The cast product according to claim 24, wherein 80 vol % or more of acrystal structure of the aluminum oxide is a corundum structure.
 26. Thecast product according to claim 24, wherein at least Cr forms a solidsolution in the aluminum oxide, and the Cr forming the solid solutionwith Al is contained so as to satisfy a relationship Al/Cr≧10 in anatomic % ratio.
 27. The cast product according to claim 24, wherein atleast one of Ni, Si, and Fe forms a solid solution in the aluminumoxide, and a total atomic % of the at least one of Ni, Si, and Feforming the solid solution with Al is 10 atm % or less.
 28. The castproduct according to claim 24, wherein the aluminum oxide has aconcentrated-Al layer in which the Al/(Cr+Ni+Si+Fe) on the matrix sideis larger than that on the surface side.
 29. The cast product accordingto claim 28, wherein a thickness of the concentrated-Al layer is onefifth or more of a thickness of the alumina barrier layer.
 30. The castproduct according to claim 24, wherein the alumina barrier layer has asurface roughness (Ra) of 15 μm or less.
 31. The cast product accordingto claim 24, wherein the matrix includes C in an amount of 0.05 to 0.7mass %, Si in an amount of more than 0 mass % to 2.5 mass % or less, Mnin an amount of more than 0 mass % to 3.0 mass % or less, Cr in anamount of 15.0 to 50.0 mass %, Ni in an amount of 18.0 to 70.0 mass %,Al in an amount of 1.0 to 5.0 mass %, rare earth elements in an amountof 0.005 to 0.4 mass %, and Win an amount of 0.5 to 10.0 mass % and/orMo in an amount of 0.1 to 5.0 mass %, and the balance includes Fe andinevitable impurities.
 32. The cast product according to claim 31,wherein the matrix further includes at least one selected from the groupconsisting of Ti in an amount of 0.01 to 0.6 mass %, Zr in an amount of0.01 to 0.6 mass %, and Nb in an amount of 0.1 to 1.8 mass %.
 33. Thecast product according to claim 31, wherein the matrix further includesB in an amount of more than 0 mass % to 0.1 mass % or less.
 34. Areaction tube made of the cast product according to claim 24, thealumina barrier layer being formed on an inner surface of the tubethrough which hydrocarbon gas as a raw material flows.